A number of devices are known for determining the viscosity of liquids. Most conventional viscometers require closely controlled conditions, such as those found in the laboratory, and are generally unsuited for use in the field. At times, however, field testing for viscosity may be required, for example, to determine if the fuel from captured or abandoned military vehicles is suitable for use. Such testing must be performed rapidly and accurately, often by relatively unsophisticated personnel, under a wide variety of temperature conditions. The test equipment is often subjected to rough handling. Unfortunately, prior techniques for measuring viscosity do not address these problems and have proven to be unduly time consuming, complicated and unreliable for use in the field.
One known method involves timing the descent of balls through respective columns containing liquids of varying viscosities. The apparatus used for this test requires extremely close tolerances. Even air bubbles can interfere with the descending balls and disrupt the test. To prevent the introduction of air into the liquid, the test chamber must be sealed. Nevertheless, the sealing gasket, which is often porous, tends to bleed. This may contaminate the liquid and invalidate the entire test. Moreover, the test balls can rattle such that the device is apt to crack or break when it is transported for use in the field.
A Cannon Fensky variety of viscometer requires timing the descent of a liquid column. This type of test requires a considerable amount of time, as well as close control over the test temperature. The completion of the test liquid's descent must be noted precisely. As a result, this technique requires sophisticated optical and electronic equipment.
Falling needle viscometers also require very close tolerances which can be disrupted by the presence of any debris or air bubbles in the test liquid. This mechanism, too, is highly sensitive to temperature fluctuations.
Rising bubble viscometers are also known. However, it is virtually impossible to control the size of the bubbles and, as a result, these mechanisms cannot provide an absolute measure of viscosity.
Drip cup viscometers involve timing a liquid stream from the orifice of a cup and noting the precise instant at which the stream converts to droplets. That point is often difficult and frustrating to discern. As a result, this test is often tedious and unreliable.